Air puzzle

[maplebeech]

My VC 2040 Cat has air supply problems. And they are different from what I've read about in various places here on the forum.

First, I cannot get the primary air to shut down completely. I have checked gaskets and have confirmed that the mechanism does close shut when I close it down. Still, the flame continues as if I haven't shut it down. So it would seem that there's a leak somewhere, but I cannot find it.

But here's what makes it so puzzling. When I close the damper to engage the Cat, it's almost the opposite situation. If I reduce the primary air, it will back puff. So I have to run the stove at 75% open for it continue to work without belching. I can't get it to go for a slow burn. The catalyst is clean. I've had various stoves over the last four decades, some of them with Cats, and this one is such a puzzle!

 

[EatenByLimestone]

How tall is your chimney, and what is the moisture content of your wood?

 

[maplebeech]

Moisture content is between 8 and 12 %. Best guess on the chimney is 15 feet or so. The house is a ranch, so couldn't be any more than that.

 

[skb31]

Be sure to test your moisture content on wood that’s been raised to room temp and on a freshly split face with the grain. 8% is quite dry so I’m skeptical of that number.

You won’t be able to cut the flames completely in updraft mode even with the air cut. You should definitely see an effect on the fire when you close the primary air but with damper in the open position it will still burn hot.

All of us recommend a digital meter such as the Auber AT 100 and a k type thermocouple for the cat chamber. The factory bimetal probe is just not adequate for the job.

 

[EatenByLimestone]

And press the pins in with the grain.

15 if straight up might be marginal depending on diameter, insulation level, if there are any bends, and your altitude. Internal vs external, masonry or class a makes a difference too.

 

[gthomas785]

This is how that stove behaves when it has a weak draft. Please tell us all about your chimney.

 

[maplebeech]

Thanks for the responses. Very helpful.

The chimney is in the center of the house. The woodstove is set into what once was a fireplace, so the stovepipe goes into that flue. It appears to have some fiberglass insulation surrounding it, but I can't see into it far enough. It's a straight shot up through the center chimney.

Also, I re-checked the wood with freshly split, and it's 14%.

 

[gthomas785]

Do you know if your stove hooked into a metal liner that goes all the way up? And is that liner insulated? Or does the stove pipe just connect into the masonry flue?

A couple other things I forgot to mention.
That stove is not meant to be run in bypass mode except when starting or reloading. If you are trying to reduce the air while the bypass is open, you will not have much success because of the firebox design. There is basically an unlimited amount of air that can backflow through the cat. Once you close the damper, the exhaust will flow through the catalyst in the right direction and the air lever will be able to effectively control the fire.

I have my 2040 on a chimney that is marginal - about 14 ft vertically from stovetop to cap, but with some bends that restrict the flow. I sometimes get backpuffing if I've loaded a lot of wood on a really hot fire, and turn it down fast. But that usually resolves after a few minutes. If I'm careful to turn it down more gradually, I do not usually have a problem with backpuffing.

 

[skb31]

14% is much closer to normal for well seasoned firewood so that’s good.

Do you know how many bends are in the flue? If so, what angles? Describe as much as you can and the smarter people here will be able to assist on that front.

Also, can you describe your process for loading your wood? I have a newer 2040 cat-c as a heads up and had some growing pains with it. The digital cat meter is really important. I basically run my stove off that and my stove pipe meter

 

[maplebeech]

The stove is hooked into a metal liner that goes all the way up. I can't tell if it's insulated because there's a metal plate that the liner passes through. It appears to be a single straight shot once it leaves the back of the stove.

I don't try to run it in bypass mode, but have just noted how the flame didn't seem to dampen when I reduced the air. Made me think there was a leak, so I'm happy to hear that the air can backfill through the cat. That makes sense.

I suspect I probably rush it along once I've got the cat engaged. I'll try to not be in such a hurry to reduce the air. Thanks for making it clear that this performance is typical for this stove.

 

[Woodsplitter67]

The stove is hooked into a metal liner that goes all the way up. I can't tell if it's insulated because there's a metal plate that the liner passes through. It appears to be a single straight shot once it leaves the back of the stove.

I don't try to run it in bypass mode, but have just noted how the flame didn't seem to dampen when I reduced the air. Made me think there was a leak, so I'm happy to hear that the air can backfill through the cat. That makes sense.

I suspect I probably rush it along once I've got the cat engaged. I'll try to not be in such a hurry to reduce the air. Thanks for making it clear that this performance is typical for this stove.

Back puffing is not the air backfilling the cat.

Back puffing is the build up of gasses in the firebox and igniting and you can smell some in the room from the stove leaking the smoke.

You should have control of your burn regardless of the draft. That being said you may not have ruled out the air leak yet do you have a thermometer on your stove pipe roughly 18" up the pipe from the stove top. This can help you with draft. If the stovepipe is cool it just doesn't draft well causing a back puffing issue

 

[maplebeech]

I can get a temp reading on the pipe 8 inches above the stove, but it disappears into the chimney after that. At 8 inches, the pipe temp is 175 while the griddle temp is 350. That's with Cat engaged.

I don't know if you would consider 175 cool.

 

[gthomas785]

Back puffing is not the air backfilling the cat.

Back puffing is the build up of gasses in the firebox and igniting and you can smell some in the room from the stove leaking the smoke.

No the backpuffing is not due to the air back feeding. I was saying that's the reason you can't turn the fire down with the bypass open. It's a quirk of this specific stove design.
Backpuffing typically occurs anytime you happen to have a really hot/active fire, combined with insufficient draft. That combination will only happen under certain circumstances and some setups are more prone to it than others. Turning the air down slower allows the chimney to build more heat/draft while also allowing the fire to slow down a bit which should reduce backpuffing.

I don't know if you would consider 175 cool.

175 degree F is definitely lower than I'd expect to see during an active burn. Is that single wall pipe, surface temp?

 

[maplebeech]

Yes, single wall and surface temp.

I just checked it after reloading. It's 485 on the griddle and 210 on the pipe.

 

[Mreif]

Yes, single wall and surface temp.

I just checked it after reloading. It's 485 on the griddle and 210 on the pipe.

That seems low, my flue temp is usually around 600* thats internal temp on double wall 18" up on pipe. I have heard your internal temp is double what your surface temp is on single wall pipe readings.

 

[gthomas785]

Yeah 210 still seems pretty low. How are you measuring it? If you're using an IR meter on shiny metal it may not be accurate.

 

[Woodsplitter67]

No the backpuffing is not due to the air back feeding. I was saying that's the reason you can't turn the fire down with the bypass open. It's a quirk of this specific stove design.
Backpuffing typically occurs anytime you happen to have a really hot/active fire, combined with insufficient draft. That combination will only happen under certain circumstances and some setups are more prone to it than others. Turning the air down slower allows the chimney to build more heat/draft while also allowing the fire to slow down a bit which should reduce backpuffing.



175 degree F is definitely lower than I'd expect to see during an active burn. Is that single wall pipe, surface temp?

I know exactly what back puffing is and expected it correctly.

You should probably reread what I sent and to who it went to

 

[gthomas785]

I know exactly what back puffing is and expected it correctly.

You should probably reread what I sent and to who it went to

the message you originally replied to was not about backpuffing. I was just trying to clarify that. Sorry if unclear

 

[maplebeech]

We're all in agreement about what backpuffing is, and I have gotten a handle on controlling it by reducing the air gradually.

Yes, I'm measuring with an IR, but I trust the numbers because I also have a thermometer attached to the cast iron just before the stovepipe elbow, and it give me similar readings.

So, clearly, I have a weak draft. What, if anything, can I do about it? I see references to plugging holes. Where are these holes, and would that do anything to increase the draft?

Thanks to all who have participated in this discussion. It's been very helpful.

 

[Woodsplitter67]

So normally the higher the stove pipe temperature the better the draft is. This in conjunction with outside temperature. The colder outside the more you will draft.

Before making any significant changes I'd work on basic operation. You can increase your draft and how the stove runs by simply keeping an eye on temperatures.

For example if my cat temperature is say 550(active) but my stovepipe temperature is 180 degrees id keep the damper open until I get to my normal stovepipe operating temperature for good draft.

You can learn what the stove/setup likes and this will help you with running the stove.

So what om saying is you will learn what you can and can't do with operating your stove. Keep an eye on your gear. I know when I start my stove with a cold start I look at STT and stove pipe, after it's running and cats engaged and I'm reloading it's cat temperature and stove pipe

 

[maplebeech]

I've received great advice here, and I've made changes to the way I operate my stove. And the upshot is that I can get it to the point where I can shut down the air completely and not get any backpuffing. So many thanks!

Various posters refer to their cat sensors as the best way to monitor the stove's performance, but the way my stove sits in the fireplace means I couldn't read a sensor on the back of the stove. Photo attached. I would like to use a cat sensor. Any advice ?

 

[gthomas785]

Get a thermocouple probe. I use the Auber AT100 with a K type thermocouple. The probe goes in the hole on the back of the stove but has a 6' cable that allows you to put the display wherever it's convenient.

 

[maplebeech]

Thanks! I'll look for it.